1. Field of the Invention
The present invention generally relates to an electromechanical transducer element, a method of manufacturing the electromechanical transducer element, a liquid droplet discharge head having the electromechanical transducer element, and a liquid droplet discharge device having the liquid droplet discharge head.
2. Description of the Related Art
As a configuration of an inkjet recording device and a liquid droplet discharge head used in an image recording apparatus or an image forming apparatus such as a printer, a facsimile machine, a copier, etc., there is a known configuration as illustrated in FIG. 1. FIG. 1 schematically illustrates an example configuration of a liquid droplet discharge head.
As illustrated in FIG. 1, the liquid droplet discharge head includes a nozzle plate 103 having a nozzle 102 formed (fabricated) therein, so that ink droplets are discharged through the nozzle 102, a compression chamber substrate 104 (Si substrate), and a base 105, so that a compression chamber 101 is defined by the nozzle plate 103 (as the lower surface), the compression chamber substrate 104 (as the side surfaces), and the base 105 (as the upper surface). Further, as a known discharge drive means to drive the discharge of ink droplets, there are, for example, an electromechanical transducer element 106 such as a piezoelectric element, an electro-thermal transducer such as a heater, and a means including a vibration plate forming a wall surface of an ink flow path and an electrode facing the vibration plate. Ink droplets can be discharged from the nozzle 102 by applying pressure to ink in the compression chamber 101 using energy that is generated by such an energy generation means described above. As such an energy generation means in an inkjet recording head, there are two types of practical means. One means is to use a piezoelectric actuator having a longitudinal (axial) vibration mode so as to extend (elongate) and contract (shrink) in the axis direction of the piezoelectric element. The other means is to use a piezoelectric actuator having a flexural oscillation mode.
Various types of inkjet recording heads using such an actuator having the flexural oscillation mode are known. For example, an electromechanical transducer element is formed as if it is independently formed for each of the pressure generation chambers by forming a uniform piezoelectric material layer across the entire surface of the vibration plate using a coating technique and cutting the piezoelectric material in the shapes corresponding to the pressure generation chambers using a lithography technique.
Further, as an example of an electromechanical transducer element used for the actuator having the flexural oscillation mode, there is an electromechanical transducer element formed of a lower electrode 106, which is a common electrode, a PZT film (piezoelectric body layer; electromechanical transducer film 107) formed on the lower electrode 106, and an upper electrode 108 which is an individual electrode formed on the PZT film. Further, an interlayer insulation film is formed on the upper electrode 108 to provide insulation between the lower electrode 106 and the upper electrode 108. Further, a contact hole is formed through the interlayer insulation film as an opening so that a wiring is electrically connected to the upper electrode 108 through the contact hole (see, for example, Japanese Patent Nos. 3365485 and 4218309).
However, as the lower electrode 106, a metal electrode in which Pt is mainly used as the base thereof is widely used. Therefore, there is a concern that it is difficult to provide assurance against the fatigue characteristics of PZT. This is because it is generally thought that the characteristics of PZT are degraded due to diffusion of Pb included in PZT. It is also known that the fatigue characteristics of PZT can be improved when an oxide electrode is used (see, for example, Japanese Patent No. 3019845).
FIGS. 2A and 2B illustrate polarization states of the PZT membrane before and after the polarization process is performed, respectively. As illustrated in FIGS. 2A and 2B, the polarization directions 21 in the piezoelectric body crystal just before voltage application are random (see FIG. 2A before the polarization process is performed). However, when a voltage is repeatedly applied to the piezoelectric body crystal, the polarization directions in most domains 20 are directed in the same direction, so that a set of domains 20 having the same polarization direction can be formed (see FIG. 2B after the polarization process is performed). In this regard, various trials have been conducted to align the directions of the polarizations before the voltage is applied thereto to stabilize a displacement relative to a predetermined drive voltage (see, for example, Japanese Laid-open Patent Publication Nos. 2004-202849 and 2010-34154). This process may be called as an aging process or a poling process (polarization process). Specifically, a method has been conducted of applying a high voltage exceeding a drive pulse voltage to the piezoelectric element.